This invention relates to a housing for a power assist apparatus having internal strut members to maintain the space relationship between a front shell and a rear shell. The struts assure that a movable wall of the power assist apparatus can move a fixed distance from a rest position to a maximum actuating position without engaging the front shell.
In order to reduce distortion that can occur when reactionary forces are transferred through the housing of a power assist apparatus, it has been disclosed in U.S. Pat. No. 4,270,353 to provide force transmitting members that extends from a master cylinder into a stationary mounting. This housing no longer is required to carry reaction forces, therefore the material that the housing is made of can be changed from a metal to a plastic. In addition, the gauge of the material can also be reduced. Unfortunately with light weight material it is possible to distort the housing and especially when the apparatus is in the rest condition. In the rest condition, the interior of the apparatus is connected to a source of vacuum while the exterior of the housing is subjected to atmospheric pressure resulting in a pressure differential being created across the housing. As long as the temperature of the housing remains below 120.degree. F. the housing retains its shape. Unfortunately, when the temperature increases above 120.degree. F. as often occurs when power assist apparatus are located in vehicles, the plasticity of the housing material allows the pressure differential to create a crushing force that attempts to move the end shells toward each other. Any increase in the thickness of the housing material to resist the crushing force substantially increases the weight of the power assist apparatus.
In an effort to further reduce the weight of a power assist apparatus it has been disclosed in U.S. Pat. application No. 437,940 filed Nov. 1, 1982 to mount a master cylinder on the stationary fire wall and have the power assist apparatus pull the pistons in the master cylinder. This structure should weigh less than the power assist apparatus disclosed in U.S. Pat. No. 4,270,353 since the force transmitting means has been eliminated. Unfortunately as with the housings disclosed in both U.S. Pat. No. 4,270,353 and U.S. Application No. 437,940, the shells are made of a light weight and thin gauge steel or plastic which when subjected to high temperature and a pressure differential created between the atmosphere and vacuum in the housing, can distort.